Control devices of this type known in the related art include the one disclosed in Japanese Patent Registration No. 2634330.
The device disclosed in this publication sets a rotation speed according to an operation amount of a rotation speed setting unit (a fuel lever) and a rotation speed according to an extent to which a travel pedal is operated, and selects a maximum value as a target rotation speed. As a result, a lowest value of the target rotation speed is restricted to a rotation speed in correspondence to an operation amount of the rotation speed setting unit so that if a rotation speed setting is set by the rotation speed setting unit to a value suitable for working (e.g., excavation), it is possible to minimize a fluctuation in the engine rotation speed and to improve the operability. Moreover, by setting the rotation speed setting at the rotation speed setting unit to the idling rotation speed during traveling, the prime mover rotation speed changes in accordance with the operation amount of the travel pedal, and thus the improvement of the fuel efficiency and the noise reduction can be achieved.
When utilizing the device disclosed in the above mentioned publication, it is necessary to maintain the travel pedal at a position depressed halfway down in order for a vehicle, for example, to travel at a constant speed since the traveling speed of the vehicle is adjusted by adjusting the operation amount of the travel pedal. However, it is a burden on an operator to keep the travel pedal at the halfway-down position.